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The Abu Walaa ISIS recruiting network has been linked to the Berlin truck attack

Analysts say as many as 20 Germans who have joined ISIS have ties to this network

(CNN)New light has been shed on the German terrorist network behind the Berlin attack in 345 pages of German investigative files that CNN has obtained.

They reveal that Anis Amri, the Tunisian suspect in the truck attack, was part of an ISIS recruitment network that orbited around a radical preacher called Abu Walaa from al-Tamim, Iraq. The cleric, who styled himself as ISIS' representative to Germany, built up a national following among Islamist extremists in the European country.

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The network even came up with a common curriculum to brainwash youngsters and persuade them to join ISIS. Their recruitment efforts were mainly based in the North Rhine-Westphalia and Lower Saxony regions of Germany, and they ran seminars in the towns of Dortmund, Hildesheim and Duisburg, which extremists from across the country visited. Supporters also downloaded Internet videos in which Abu Walaa's face was not featured, giving him the nickname the preacher without a face.

Just like the so-called Zerkani ISIS recruiting network in Brussels, Belgium, the German network raised funds for jihadi travel through robberies. Italso raised money through applications for fraudulent loans.

Another leading figure in the Abu Walaa network was Boban Simeonovic, a 36-year-old German-Serbian from Dortmund who acted as a guru to Amri and had a reputation for fanaticism even in these radical circles.

Simeonovic stressed to followers in Germany that there was a "state of war" in the country and attacks were necessary, according to the investigative files.

Suspect Anis Amri was part of an ISIS recruitment network called Abu Walaa, investigative files show.

Both Abu Walaa and Simeonovic were arrested in November, along with three other leading figures in the network. They were all charged with terrorism offenses.

When Amri was preparing to travel to join ISIS in late 2015, Simeonovic took him and other jihadi wannabes on long hikes with backpacks to get them in shape and tried to arrange for him to leave the countrythrough his contacts at a mosque in Hildelsheim. It is not clear why he was not able to travel.

While in Germany, Amri communicated with the radical network using the encryption app Telegram, according to the investigative files.

Photos:Berlin attack aftermath

The Brandenburg Gate in Berlin is illuminated in the colors of the German flag on Tuesday, December 20, one day after a truck crashed into a crowded Christmas market there. At least 12 people were killed and 48 injured in what police are investigating as a terrorist attack.

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A police officer places a candle at a makeshift memorial in Berlin on December 20.

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A woman is illuminated by candles as she cries in Berlin on December 20.

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A man prays at a Berlin memorial on December 20.

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German Chancellor Angela Merkel lays flowers at the memorial on December 20. She is joined by, from left, Berlin Mayor Michael Muller, Minister of the Interior Thomas de Maiziere and Foreign Minister Frank-Walter Steinmeier.

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Merkel, bottom left, views the scene of the attack.

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A woman reacts near the crime scene on December 20.

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Mourners gather at the Kaiser Wilhelm Memorial Church near the crash site.

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A police officer holds her weapon near the market.

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A woman lays flowers at a makeshift memorial across from the market.

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A stall worker is comforted in the market area on December 20.

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People attend a memorial service at St. Hedwig Cathedral in Berlin on December 20.

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A police van drives by Christmas decorations at a Christmas market in Frankfurt, Germany, on December 20. Police presence has been stepped up at Christmas markets across Germany following the attack in Berlin.

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Sometime before or after his travel plans fell through, Amri started discussing launching an attack in Germany, according to a police informant in the network who was cited in the investigative files. Simeonovic was in favor of this and gave the Tunisian a place to hide, the police informant told German investigators.

Other members of the Abu Walaa network discussed driving a truck full of gasoline with a bomb into a crowd, the police informant told investigators, according to the investigative files.

According to the investigative files, handwritten notes were found indicating that Simeonovic had direct contact with a number of German ISIS operatives in Syria, including the German convert Christian Emde and Silvio Koblitz, a recruit from Essen.

The investigative files also include information from Anil O., a German-Turkish defector from ISIS tied to the network, now cooperating with German authorities. The defector claimed Abu Walaa had particular clout in Raqqa, Syria, because one of his former acolytes, a German ISIS operative named Lemke, was in charge of German members of the group's Amniyat security service and was in touch with Abu Muhammad al-Adnani, who until his death was the second most powerful figure in ISIS.